How do you use a moving average to filter out accelerometer values in iPhone OS

Question

I want to filter the accelerometer values using a moving average, how is this done? Thanks

Answer 1

A simple, single pole, low pass, recursive IIR filter is quick and easy to implement, e.g.

xf = k * xf + (1.0 - k) * x;
yf = k * yf + (1.0 - k) * y;

where x, y are the raw (unfiltered) X/Y accelerometer signals, xf, yf are the filtered output signals, and k determines the time constant of the filters (typically a value between 0.9 and 0.9999..., where a bigger k means a longer time constant).

You can determine k empirically, or if you know your required cut-off frequency, Fc, then you can use the formula:

k = 1 - exp(-2.0 * PI * Fc / Fs)

where Fs is the sample rate.

Note that xf, yf are the previous values of the output signal on the RHS, and the new output values on the LHS of the expression above.

Note also that we are assuming here that you will be sampling the accelerometer signals at regular time intervals, e.g. every 10 ms. The time constant will be a function both of k and of this sampling interval.